Now those jobs and what's done at the Kaesong complex are in the international spotlight. On Wednesday, North Korean authorities blocked trucks and workers coming from the South to the complex about six miles inside North Korea.

As NPR's Louisa Lim told Morning Edition, North Korea's regime is skilled at "this sort of cycle of threats" — particularly at times, such as now, when the U.S. and South Korea are holding joint military exercises.

So, along with threats to fire missiles as the South and the U.S., leader Kim Jong Un and his generals have also shown their displeasure by cutting one of the last ties between North and South — the access given to companies from the South to workers at the Kaesong complex.

Experts hope the move is just the latest in decades of rhetoric from the North and that South Korean companies will have access to Kaesong again soon.

Meanwhile, here's more about the complex:

-- The project was launched in 2003 in the hope it would both provide much-needed income for those in the North and build better relations with the South. Work began there the next year, according to Foster-Carter. Now, reports the BBC, there are 123 companies from the South with operations in the complex.

-- Along with small appliances, the companies with operations in the complex make clothing, textiles, car parts and semiconductors. About $470 million worth of goods were produced there last year, the BBC says.

-- Everything made there is exported to the South.

-- The South Korean government has not only given companies incentives to put operations at Kaesong, it has also made available "political risk insurance" to cover any losses if North-South relations sour further.

-- About 800 South Koreans are at the complex most workdays.

-- A "Mr. Kim ... [who] asked that his full name not be used," is a manager for a South Korean sportswear company that employs about 950 North Koreans at the complex. "The skill and labor intensity of workers at Kaesong is far better than we could get in China or Vietnam," he tells The Wall Street Journal. "They're disciplined, hard workers and of course language is no problem."